A tiny leak inside the Unit 3 reactor at Palo Verde Nuclear Generating Station could delay reopening of the unit as long as 30 days and cost Arizona Public Service Co. and the other plant owners $10 million to $15 million to repair, officials said.

The leak released a small but unknown amount of radioactive water inside the concrete containment dome over the reactor, but not to the outside environment. The leak has not threatened any of the plant’s nearly 3,000 workers, officials said.

The leak was discovered by remote cameras after the unit was shut down Oct. 5 for scheduled refueling. Each of the three reactors 50 miles west of Pheonix is powered down every 18 months to replace about one-third of the fuel inside and to conduct scheduled maintenance.

The leak is on the bottom of the reactor vessel, where water is heated by the uranium fuel. No water was present, but a white residue indicating boric acid, which is used in the water, hinted that a leak occurred.

“Think about taking your car in to the mechanic on its 50,000 mile check up, that’s what we do,” said Bob Bement, senior vice president of site operations at Palo Verde. “We go under the hood and look at everything and make sure it is set up for long term operation.”

Officials were unsure how much water leaked out, because it would have immediately flashed to steam upon leaking. But they said that instruments constantly measure the humidity in the area, and those monitors did not detect an uptick in moisture in the air during the unit’s most recent run, nor did their monitors detect a drop in the amount of water in the vessel. They suspect the leak was less than 1/100th of a gallon per minute.

Palo Verde is operated by APS on behalf of the seven utilities from Texas to California that own a portion of the plant. The officials were anticipating the scheduled outage to take about 30 days, but the complicated repair work on the leak could increase that by 15 to 30 days, Bement said.

APS’ main goal is to repair the leak properly to safely generate electricity for the long term, Bement said, so the company will repair the leak properly regardless how long the plant must be kept offline.

Palo Verde generates more electricity every year than any other power plant in the country, a record it is on track to maintain for the 22nd consecutive year.

The delay means APS and other plant owners will have to find replacement power in the meantime. The unit averages about 31,488 megawatt-hours of output a day when it is running.

One megawatt-hour of electricity was trading for between about $30 and $40 in the regions from Texas to Southern California, including Arizona, as of last week, according to the Energy Information Administration. That means that the power produced from one unit of Palo Verde is worth about $1 million a day or more to the plant owners.

“We are going to put safety and reliability first, in that order,” APS spokesman Jim McDonald said. “You are not going to rush the outage with production in mind. This is not a race against the clock.”

The spring and fall outages are timed at Palo Verde to coincide with periods when electricity demand and prices are low, he said.

The other plant owners are Salt River Project, Southern California Edison Co., El Paso Electric, Public Service Co. of New Mexico, Southern California Public Power Authority and the Los Angeles Department of Water & Power.

In addition to the repair, Palo Verde has been adding about $100 million in upgrades to all three reactors to make them more resilient to large-scale disasters such as the Fukushima Dai-ichi plant in Japan.

The March 11, 2011, earthquake and tsunami in Japan knocked out power to a six-reactor nuclear facility. Two reactors were in shutdown mode, and a third had been de-fueled. But without power, the three reactors that were running eventually melted down when they could not be kept cool with recirculated water. The used fuel in the other reactors also needed to be kept cool with water.

APS officials said the threat of flooding at the Arizona plant is remote, but they conducted some earthwork on the plant site to prevent any water from backing up to the reactor site.

They also are adding hook-ups for outside diesel generators, should they ever need to be brought in if the plant loses outside power like the plants in Japan. Similar connections are being added to the spent fuel pools at Palo Verde, in addition to new pumps that can be brought in if the plant’s pumps fail.

“This gives our operators equipment they will never need, we hope,” Bement said.

The upgrades will continue to be added to the reactors during their scheduled refueling outages, and should be complete by 2015, he said.

A reactor unit at the Palo Verde Nuclear Generating Station in Arizona was nearly back to full production Friday after what its operator described as a minor explosion in a cabinet that held electrical switching gear.

Arizona Public Service Co. reported the Tuesday night incident as an “unusual event,” the lowest of four emergency levels classified by the federal Nuclear Regulatory Commission. The company and an inspector with the regulatory commission said no radioactive material was released.

Palo Verde’s Unit 1 automatically reduced its power production to 60 percent but started ramping back up Thursday, reaching 95 percent by early Friday afternoon.

APS spokesman Betty Dayyo said the explosion consisted of arcing inside a cabinet, called a “load center,” that routes electricity to equipment in a turbine building. “This is really a minor incident,” said Dayyo, adding the cause was under investigation.

Palo Verde’s own fire department responded but found no fire, APS said.

Arizona, California, New Mexico and Texas utilities own the three-reactor plant, 50 miles west of downtown Phoenix.

John Reynoso, the Nuclear Regulatory Commission’s senior on-site inspector for Palo Verde, said he went to the plant right after the evening incident and inspected the load center.

He said it was blackened by an apparent flash-over but said he saw no immediate indication of shortcomings that would indicate a broader problem.

NRC inspectors will monitor the operator’s investigation, Reynoso said. “We’re real interested in how they learn from this.”

APS and another Arizona utility, the Salt River Project, own the plant along with Public Service Co. of New Mexico, El Paso (Texas) Electric Co, Southern California Public Power Authority, Southern California Edison Co. and the Los Angeles Department of Water and Power.